This project investigates the ways in which participation in social interactions change brain systems associated with reproductive function, and the hormones and behavior they control. Following previous work the project will focus on three forebrain systems, the gonadotropin releasing hormone cells that directly control the pituitary and its regulation of gonadal steroids, tyrosine hydroxylase (dopaminergic) containing cells in several locations that participate in several processes related to reward, motor control, and several aspects of male sexual behavior and endocrine control, and arginine vasotocin ceils which comprise an important neuromodulator system influencing social behavior and communication. The project uses a combination of behavioral tests, endocrinological manipulations, and neuroanatomical methods (particularly immunocytochemistry), tied together with statistical model testing using Structural Equation Modeling, to test several hypotheses about the way in which exposure to communication signals associated with sociosexual behavior sculpt brain systems. It will test the specific hypothesis that sex steroid changes triggered by social stimulation is the critical, mediating factor in inducing the neural changes resulting from such stimulation and examine alternative mechanistic routes by which androgens might act. It will also test two hypotheses concerning the mechanisms underlying the neural changes: that the resultant neural changes involve activation of the pCREB signaling pathway that has been demonstrated to be important in other brain areas for mediating neural plasticity associated with cognitive memory, and/or that the neural changes reflect an increase in neurogenesis. Furthermore, it will compare changes induced by social stimulation to those induced by changes in hormonal state independent of social cues. Supporting the hypothesis driven portions of the project will be more descriptive studies identifying the location of androgen receptors and aromatase enzymes so as to provide neuroanatomical data with which to interpret the hormonal effects on the three target populations and more generally on any observed patterns of pCREB formation and neurogenesis up-regulation. Lastly, the data obtained in different portions of the project will be employed to test alternative hypotheses about the causal relationships among the brain changes, the hormonal changes, and the behavioral changes induced by social stimulation. The project will enhance the understanding of the ways in which social interactions change the levels of circulating sex and stress hormones in an individual, and how these changes may in turn affect the brain systems important for controlling sexual behavior, aggression, and endocrine regulation. These are all natural behaviors with clinical significance, as dysfunctions of sexual response and reproductive function, affiliation and aggression, and stress are important human clinical problems.